This invention relates generally to the measurement of radioactivity utilizing what is generally referred to as the coincidence counting technique, wherein events relating to radioactive decay are detected in two or more detectors within a given time interval, in order to eliminate various sources of error which would be introduced if only one detector were used. More particularly, the invention relates to a new and improved method of eliminating a further source of error due to the detection of chance or random coincidences in the multiple detectors.
One of the most widely used devices for the measurement of radiation from radioactive substances is the scintillation counter. The basic element of a scintillation counter is a scintillation medium which absorbs incident radiation and emits photons as a result. Many of the emitted photons are incident upon a photocathode in a nearby multiplier phototube, and are converted to photoelectrons emitted from the photocathode. The electrons emitted from the photocathode are multiplied in number at a succession of phototube electrodes, called dynodes, and the output of the multiplier phototube is a measurable electrical pulse having a magnitude which is approximately proportional to the energy of the incident radiation.
A liquid scintillation counter operates on the same basic principle, except that the scintillation medium is a liquid into which is dissolved, suspended or otherwise intermixed the sample being tested. The radioactivity of the sample can then be measured by collecting the photons emitted from the scintillation medium in a multiplier phototube positioned near the sample, and counting the pulses generated by the tube in appropriate electrical circuitry. Depending on the characteristics of the sample being measured, and on the particular tests being performed, this electrical circuitry usually performs some type of pulse height analysis on the pulse output from the phototube, since it is usually desired to detect and count radioactive events within a particular range or window of energy levels.
A significant problem encountered in the measurement of radioactivity by means of scintillation counters is that there are a number of phenomena unrelated to the radioactivity of the sample which nevertheless result in the generation of output pulses from the multiplier phototube of a scintillation counter. These phenomena are frequently referred to as "singles" events since they are all characterized by the emission of single photons or photoelectrons. A relatively large source of "singles" events is the thermionic emission of electrons from the photocathode or from the dynodes of the multiplier phototube itself. Such electrons are emitted independently of any detected radiation, and can result in significant error, especially if the tubes are operated at relatively high voltages, the typical case where high amplification factors are being used, as in the measurement of relatively low radiation levels. This thermionic emission of electrons is also referred to as "tube noise."
In liquid scintillation counters, the sample itself may emit photons by some process unrelated to its radioactivity. The sample material could exhibit some degree of chemiluminescence, i.e., there may be some chemical reaction or reactions ocurring within the sample material which result in the emission of photons. The sample material may also be subject to the processes of bioluminescence or photoluminescence, which also generate photons independently of the level of radioactivity of the sample material. In addition, the presence of low-level background radiation, static electrical discharges, or a leakage of ambient light into the counter, could give rise to "singles" events detectable by the scintillation counter.
Use of the well known coincidence counting technique substantially reduces the detection of "singles" events by a scintillation counter. In a liquid scintillation counter this technique is utilized by employing at least two multiplier phototubes disposed one on each side of the sample. The emission of many single radioactive particles by the sample can typically result in the emission of about seven or more photons simultaneously, or nearly simultaneously. Thus, there is a high probability that such an event will be detected by both phototubes at nearly the same time. A "singles" event, however, such as one resulting from chemiluminescence, or from a thermionically emitted electron in one of the tubes, would result in an output pulse from only one of the tubes. It can be appreciated, then, that the use of the coincidence counting technique results in the elimination of most of the "singles" events from the counting process.
It will also be apparent, however, that, because of the random nature of the "singles" events, there is a significant probability that a "singles" event could be detected in one tube at nearly the same instant in time that one is detected in the other tube. There is, therefore, a random coincidence rate resulting from random or chance coincidences of unrelated "singles" events. Mathematically, the random coincidence count rate S.sub.c is given by: EQU S.sub.c = 2.tau..sub.c S.sub.1 S.sub.2, (1)
where:
.tau..sub.c = the resolving time of the coincidence counter, i.e., the longest time separating two pulses which would still be considered coincident, PA1 S.sub.1 = the "singles" count rate measured by one of the multiplier phototubes, and PA1 S.sub.2 = the "singles" count rate measured by the other of the multiplier phototubes.
Under normal operation, a liquid scintillation counter will give a measured count rate (S.sub.m) which will be the sum of the sample coincidence count rate (S.sub.a) and the "singles" random coincidence count rate (S.sub.c). That is: EQU S.sub.m = S.sub.a + S.sub.c. (2)
Any user of a liquid scintillation counter ideally needs to know the value of S.sub.c so that, where possible, a correction can be made to obtain the correct value of radiation attributable to the sample only. Even where direct correction is not possible, because testing is being performed in specific energy level "windows," the user of the counter could still use the value of S.sub.c as an indication of the reliability of the measured count rate.
Prior to this invention, a precise determination of the random coincidence rate has not been possible. It has only been possible to make a qualitative estimate of the presence of those "singles" events which decrease with time. Usually chemiluminescence, bioluminescence and photoluminescence have this decay characteristic. In accordance with such prior art techniques, the radioactivity of a sample would be measured at different times, and the measured count rates compared so that any decrease in the measured coincidence rates could be noted. If there was little or no decrease in the measured coincidence rates over a substantial time period, it was generally assumed that the random coincidence count was insignificant. This method is, of course, quite time consuming, and does not take into account at all those "singles" events derived from tube noise, or from other sources which do not rapidly decay.
Accordingly, there is a clear need in the field of radioactivity measurement for a coincidence counting technique which provides a reliable estimate of the error attributable to random or chance coincidences of "singles" events, thereby enabling more accurate measurements of radioactivity. The present invention fulfils this need.